A helicopter rotor hub is the primary structural assembly for driving torque to and reacting the centrifugal loads of each rotor blade and transferring lift loads thereof to the aircraft fuselage. Common varieties of rotor hubs include articulated, hingeless and bearingless types wherein the rotor hub is characterized by the specific means for accommodating the multi-directional displacement of the rotor blades. For example, articulated rotor hubs typically employ one or more bearing elements to accommodate rotor blade excursions whereas bearingless rotor hubs utilize flexible structures, commonly termed "flexbeams", to functionally replace the bearing elements of articulated rotor hubs.
Within the class or category of articulated rotors are those which include a central hub member for driving a plurality of rotor blade assemblies via spherical multi-laminate elastomeric bearings. More specifically, the hub member includes a plurality of radial spokes and shear segments which structurally interconnect a pair of radial spokes. Each shear segment, in combination with its respective pair of spokes, forms a structural loop which, depending upon the configuration of the hub member, may be vertically or horizontally oriented. Each structural loop accepts a rotor assembly yoke which is generally C-shaped and circumscribes, in looped fashion, the shear segments of the hub member. The rotor assembly yoke includes a midsection, which extends though the respective structural loop, and a pair of radial arms which are disposed on either side of the shear segment. The proximal ends of the yoke arms mount to the root end of the respective rotor blade or, alternatively, to an intermediate cuff structure. A spherical elastomeric bearing comprised of alternating layers of elastomer and nonresilient shims is interposed between the midsection of each yoke and the shear segment to accommodate the loads and motions of the associated rotor blade.
Centrifugal forces are transferred to the hub member as a compressive load in the elastomeric bearing, i.e., as the yoke bears against the innermost bearing endplate of the elastomeric bearing. The spherical configuration of the elastomeric bearing accommodates the transmission of torque to the rotor blade, provides for the transmission of lift loads to the rotor hub and, accommodates the in-plane (edgewise), out-of-plane (flapwise) and pitch change (feathering) motion of the rotor blade. U.S. Pat. Nos. 3,761,199, 4,235,570, 4,568,245, 4,797,064, and 4,930,983 illustrate articulated rotors of the type described above and are generally indicative of the current state-of-the art.
The arrangement for mounting the rotor blade assembly to the rotor hub assembly, i.e., the root end of the rotor blade to the rotor assembly yoke, often requires that a twist disparity therebetween be reconciled. Such twist disparity arises inasmuch as the mounting lugs/devis arms of the rotor yoke are typically oriented in a horizontal plane while the root end of the rotor blade is typically canted with respect thereto. With respect to the latter, it is generally advantageous to effect a high angle of attack at the inboard section of a rotor blade for maximizing lift and generating a uniform downwash. Depending upon the desired aerodynamic properties the rotor blade assembly, the inboard angle of attack may exceed 20 degrees relative to the rotor assembly yoke.
Prior art mounting arrangements typically reconcile the twist disparity by means of an intermediate metallic cuff structure wherein the flanges thereof are canted with respect to the inboard lugs/clevis arms. While such cuff structures are readily fabricated from metallic materials having substantially isotropic strength properties, the complex geometry thereof does not facilitate fabrication utilizing composite materials, i.e., fiber reinforced resin matrix materials, wherein strength thereof is highly directional. It will be appreciated that manufacturing difficulties arise when attempting to arrange the fibers in the proper orientation to accommodate the various load paths through the cuff structure.
Other mounting arrangements for accommodating the twist disparity include the formation of a twist reversal at the rotor blade root end such that the blade lugs/clevis arms are coplanar with those of the rotor assembly yoke. While such mounting arrangements eliminate the requirement for an intermediate cuff structure and obviate the attendant weight penalties associated therewith, the twist reversal complicates the tooling employed in the fabrication of the blade assembly and creates difficulties associated with optimally orienting the fiber reinforcement thereof for optimum structural efficiency. With regard to the former, the introduction of twist in the fabrication of a composite tubular structure such as a rotor blade often requires the use of multi-element tooling, i.e., mandrel assemblies, to obviate internal tool lock caused by the twist geometry of the rotor blade. With regard to the latter, optimum fiber orientation is difficult to achieve when laying composite plies over a complex curvature mold or mandrel such as that required by the reversed twist geometry. This problem is more pronounced when employing low cost composite manufacturing methods, e.g., hand lay-up of composite plies, wherein the fiber orientation is caused to deviate from an optimum angle due to the complex curvature. For example, the complex curvature will cause spanwise unidirectional fibers, which are typically desirable for reacting axial and bending loads, i.e., centrifugal and flapwise/edgewise bending loads, to be "off-axis" with respect to the direction of the load vector.
In addition to the manufacturing difficulties discussed above, the twist reversal also degrades aerodynamic performance of the rotor blade assembly. Insofar as such reversed twist typically occurs over a spanwise length of about 3% to 5%, it will be appreciated that the reduced angle of attack along such length degrades the overall aerodynamic performance of the rotor blade assembly.
A need therefore exists for providing a cuff structure for accommodating the twist disparity between the root end of the rotor blade assembly and the rotor assembly yoke without degrading the aerodynamic performance of the rotor blade assembly, and which avoids tooling complexity and facilitates fabrication via low-cost manufacture methods.